"In contrast to its vast quantity, the quality of solar energy is low relative to fossil fuels.

Tell that to the street vendors in Mexico City whose ration of fossil fuels is delivered in the air they breathe.

You're confusing quality with state. Solar energy is a Field - you can't touch it, weigh it or contain it. Fossil fuels are solids, liquids or gases. (The known energy states are Solid, Liquid, Gas, and Field.)

Consider the energy flow in the Earth’s crust. The total heat loss from the Earth’s crust is 1.3*10^18 Btu per year, equivalent to nearly 4 times the world’s annual energy use. But this energy flow is spread over the entire 5.1*10^14 square meters of the Earth’s surface. This means that the amount of energy flow per unit area is 2,400 Btu per square meter, an amount equivalent to just 1/100 of a gallon of gasoline.

Consider incoming solar energy. The land area of the lower 48 United States intercepts 4.7*10^19 Btu per year, equivalent to 500 times of the nation’s annual energy use. But that energy is spread over nearly 3 million square miles of land area, so that the energy absorbed per unit area is just 1.5*10^13 Btu per square mile per year. But plants, on average, capture only about 0.1% of the solar energy reaching the Earth. This means that the actual plant biomass production in the United States is just 1.6*10^10 Btu per square mile per year.

These examples illustrate that heat flow from the Earth, solar energy, plant biomass and other renewable forms of energy are diffuse forms of energy, particularly when we compare them to fossil fuels. This is captured by the concept of power density. Power density combines two attributes of energy sources: the rate at which energy can be produced from the source and the geographic area covered by the source. A coal mine in China, for example, can produce upwards of 10,000 watts per square meter of the mine. As the above examples indicate most solar technologies have low power densities compared to fossil fuels.

Be careful how you draw your boundaries. A coal mine in China ruins the landscape far beyond the confines of the hole in the ground, just as it does in Appalachia. The power plant burning that coal sends acid rain across national borders. The CO2 can impact New Orleans, halfway around the world.

"The annual electricity-consumption of the United States is about 3,479 billion kWhr. [Not that I'm advocating dedicating the desert to solar while there are millions of rooftops available with nothing better to do, but just to put "diffuse" energy in perspective ...] PYRON solar power plants can produce this amount on 7,731 km2 (87.92 km x 87.92 km or 54.64 mi x 54.64 mi) at a yearly solar radiation of 2790 kWhr/m2."

A low energy or power density means that large amounts of capital, labor, energy and materials must be used to collect, concentrate and deliver solar energy to users. This tends to make them more expensive than fossil fuels. The difference between solar and fossil energy is best represented but their energy return on investment (EROI). The EROI for fossil fuels tends to be large while that for solar tends to be low (Figure 6).

… unless you consider oil depletion, the ecological footprint of extraction, or the inherently low amount of energy embodied in the ephemeral technology needed to exploit solar energy.

Your statement may seem intuitive, but there is no scientific basis for it. A hundred years ago, oil gushers produced with high yield, but today solar, hydro-electric and wind power have net energy yields (EROI) higher than conventional fuels (oil, gas and coal) and an order of magnitude better than non-conventional fossil fuels (enhanced oil recovery (EOR), tar sands, oil shale, coal-to-liquid, LNG).

This is the principal reason that humans aggressively developed fossil fuels in the first place.

Fossil fuels have allowed us develop lifestyles that also are very energy intensive. The places that we live, work and shop have every high power densities. Supermarkets, office buildings and private residences in industrial nations demand huge amounts of energy. This very energy-intensive way of living, working, and playing have been made possible by fossil fuels sources that are equally as concentrated.

Another quality difference between renewable fuels and fossil fuels is their energy density: the quantity of energy contained per unit mass of a fuel (Table 4). For example, wood contains 15 Mj per kilogram; oil contains up to 44 Mj per kilogram. Higher energy densities also contribute to the higher EROI for fossil fuels relative to many renewable fuels."

… and what is your rationale for assuming conversion of solar energy (a field) into fuel (a liquid) in the first place?

To meet society's needs, renewable energy doesn't need to be converted into fuel. An electric vehicle is at least twice as efficient as a gasoline vehicle, and can be substantially more than twice as efficient under the right conditions.

Table Notes: Estimates of energy return on investment (EROI) ratios for some existing and proposed fuel supply technologies. Numbers in parentheses for electricity generation include a quality factor based on a heat rate of 2,646 kcal/kWh (10,493 BTUs/kWh)

The highlighted section provides information on new developments in solar energy and data for wind energy (an apparent omission by the authors of this study).

The following graph dramatically demonstrates the significance of advances in renewable energy technology -- a preview of the transformation which is about to take place.

"Even if there were no greenhouse effect, all of the fossil fuels will be depleted within a few hundred years. If humankind is going to have a future on this planet, at least a high-technology future, with a significant population of several billions of humans continuing to inhabit the Earth, it is absolutely inevitable that we'll have to find another energy source."